We need to show our world the face of a humanity in endless growth towards love, delighted and engaged by the glory of God that we look towards. St Paul speaks (in II Corinthians 3.18) of how "with our unveiled faces reflecting the glory of the Lord", we are transfigured with a greater and greater radiance. That is the face we seek to show to our fellow human beings.

This is not because we are in search of some private "religious experience" that will make us feel secure or holy. In the early church, there was a clear understanding that we needed to advance: from the self-understanding or self-contemplation that taught us to discipline our greedy instincts and cravings, to the "natural contemplation" that perceived and venerated the wisdom of God in the order of the world, and allowed us to see created reality for what it truly was in the sight of God – rather than what it was in terms of how we might use it or dominate it.

In this perspective, contemplation is very far from being just one kind of thing that Christians do: it is the key to prayer, liturgy, art and ethics, the key to the essence of a renewed humanity that is capable of seeing the world and other subjects in the world with freedom – freedom from self-oriented, acquisitive habits and the distorted understanding that comes from them.

Contemplation is the only ultimate answer to the unreal and insane world that our financial systems, our advertising culture, and our chaotic and unexamined emotions encourage us to inhabit. To learn contemplative practice is to learn what we need so as to live truthfully and honestly and lovingly. It is a deeply revolutionary matter.

We have to be every careful in our evangelisation not simply to persuade people to apply onto God and the life of the spirit all the longings for drama, excitement and self-congratulation that we so often indulge in. The American scholar of religion, Jacob Needleman, wrote that the words of the gospel are addressed to human beings who "do not yet exist"; and responding in a life-giving way to the gospel means a transforming of our whole self, our feelings and thoughts and imaginings. To be converted to the faith does not mean simply acquiring a new set of beliefs, but becoming a new person, a person in communion with God and others through Jesus Christ.

Contemplation is an intrinsic element in this transforming process. To learn to look to God without regard to my own instant satisfaction, to learn to scrutinise and to relativise the cravings and fantasies that arise in me – this is to allow God to be God, and thus to allow the prayer of Christ – God's own relation to God – to come alive in me. Invoking the holy spirit is a matter of asking the third person of the trinity to enter my spirit and bring the clarity I need to see where I am in slavery to cravings and fantasies, and to give me patience and stillness as God's light and love penetrate my inner life. Only as this begins to happen will I be delivered from treating the gifts of God as yet another set of things I may acquire to make me happy, or to dominate other people.

And as this process unfolds, I become more free – to borrow a phrase of St Augustine (Confessions 4.7) – to "love human beings in a human way", to love them not for what they may promise me, to love them not as if they were there to provide me with lasting safety and comfort, but as fragile fellow creatures held in the love of God.

This "internal" transformation is not more important than action for justice, but without it our search for justice or for peace becomes another exercise of human will, undermined by human self-deception. The two callings are inseparable, the calling to "prayer and righteous action", as Protestant martyr Dietrich Bonhoeffer put it, writing from his prison cell in 1944. True prayer purifies the motive, true justice is the necessary work of sharing and liberating in others the humanity we have discovered in our contemplative encounter.

• This is an edited version of the talk that Rowan Williams gave to the General Synod of Bishops in Rome last week. The full address is available at www.archbishopofcanterbury.org